The present invention generally relates to an improvement in a step and repeat (copying) machine. What is meant by a step and repeat machine is a copying machine having a support table, on which a composite film or a printing plate is intended to be mounted, and a printing frame, which is movable over said support table and which has an exposure light means, and which printing frame can be moved to different positions over the composite film or the printing plate and provide a printing at said several different positions of one and the same film original or a series of different film originals. (By original is meant the negative or positive film having text and pictures and intended to be printed onto a composite film, a printing plate etc.)
In such a copying machine composite films or printing plates are prepared, for instance a film or plate for printing of a predetermined number of pages of a book, a newspaper, a catalogue etc. Thereby it is important that the impositioning of the pages is correct, so that the pages take the correct position on the composite film or the printing plate, and that all parts of the film or plate round the entire double page which do not exhibit an image are eliminated.
The invention is in the first place useful for so called positiv printing (copying), that is a printing in which the exposure light is intended to "burn out" the parts of the composite film or the printing plate which wear no images, whereas the image parts are remained after the exposure and the development of the film or the plate. The invention also may be utilized, however, for negative printing and for other purposes.
When copy printing original films by means of a step and repeat print machine it has been usual to use a printing frame having a glass plate which is, at the bottom surface thereof, formed with vacuum channels intended to vacuum (suction) connect an original film, which in connection to the printing thereof onto a composite film or a printing plate is put into intimate contact therewith, whereupon the exposure light is lit on and the original film is printed on the composite film or the printing plate.
In order to avoid illumination and light damaging of the parts of the composite film or the printing plate located outside the original film, and on which parts other original films are to be printed in the succeeding printing steps it has been usual to use a masking frame (or several masking frames), consisting of a non-transparent material, generally metal, and generally having a central opening the form and size of which corresponds to the intended area of exposure, which area varies depending on the size of the image presenting part of the original film.
The image free parts between the printing pages can easily be exposed off in that the original film is made slightly "bleeding" or larger than the image presenting surface, however not of such large size as to influence the image presenting parts of the adjacent pages. According to this method it is, however, not possible to exposure off (or burn out) the parts of the composite film or the printing plate corresponding to the free areas outside the corner pages or edge pages. This has to be done in a separate operation, whereby the previously exposed areas are masked over and the outer, still non-exposed, areas (the edges) of the composite film or the plate are exposed.
The object of the invention is to suggest a method of making any necessary exposures in one and the same operation, which means both to expose the original film and to "burn-out" (remove) the parts of the composite film or the printing plate facing outwardly towards the edges of the composite film or the printing plate.
According to the invention this is made in that the entire printing frame is made of a transparent material like glass or plastic, and in that the printing frame is, at the bottom surface thereof, formed with channels for vacuum receiving both an original and a masking film which, for the pages which are to be printed at the outermost areas of the composite film or the printing plate, cover only the inwardly facing parts, whereas said masking film leaves the outwardly facing edge or edges free for illumination, whereby consequently said outer areas of the positive composite film or plate are exposed or "burnt out" at the same time as the original film is exposed. The advantages of said printing method is that the composite film or plate can be prepared in one single process step.
In may happen, however, that the vacuum channels for vacuum connecting the original film and the masking film and provided at the bottom surface of the printing machine glass give shadows of the generally sharp exposure light and present lines or other formations on the composite film or the printing plate. For eliminating this problem it has been found necessary to arrange for such a diffusing or other modification of this part of the light passing the glass plate that shadown phenomenas are avoided at the same time as the light passing the glass plate for printing the original film onto the composite film is left uninfluenced and strong and sharp.
Therefore, according to a preferred embodiment of the invention, the parts of the glass plate located outside the image carrying area of the original film are made frosted for diffusing purposes, preferably in that the upper surface of the glass of the print machine is blasted.
It may happen that the blasted or otherwise frosted surface becomes worn by time, or that the blasting surface becomes wet at some place, so that the light passes un-diffused also through said worn or wet part, whereby the vacuum channels at the bottom surface of the glass may still give shadows. For protecting the frosted surface a cover plate of a clear, transparent material can be glued or otherwise attached on top of the printing glass. This is in particular advantageous in that the connection bores or grooves for the vacuum channels at the bottom surface of the printing glass can be cut in the form of open grooves at the upper surface of the printing glass, which grooves are sealed by the cover glass. Preferably the frosting is made at the bottom surface of the cover glass in order to completely cover the frosting of the surfaces which have no images and whereby said frosting is protected against any action.
As an alternative to frosting of the glass surface a frosting-foil can be mounted on the upper surface of the print glass or between the cover glass and the print glass. It may also be advantageous to provide the frosting foil in the form of a double-sticking tape foil which serves, at the same time, also to join the print glass and the cover glass.
It is obvious that there should be a special print frame having a glass clear print opening of special form and size for each format of the image supporting surface, and that the vacuum channels provided at the bottom surface of the print glass should be adapted to the actual format of the original film and of the masking film.
Now the invention is to be described more closely with reference to the accompanying drawings. It is, however, to be understood that the embodiments of the invention described and illustrated in the following are only illustrating examples, and that many variations and modifications may be presented within the scope of the appended claims.